This invention relates to small computers and similar electronic devices of the type that must be locked to protect information stored therein as well as expensive components, yet must permit access for repair even when the proper key is not available to the repair person.
The use of a fracture ring to allow a locked personal computer to be opened for repair is known in the art in an IBM PS/2 (R) model 50 personal computer. This ring allows access when it is broken without destroying a more expensive lock or its mounting. However, the lock can be carefully rotated back into its original position and the forced entry will not be noted until an operator again unlocks the computer at which time the break in the parts of the ring that keeps the lock from turning may become apparent. The use of a master key in this application is impractical because a low cost master keyed lock that can be opened by any repair depot can also be opened by the stolen master key without any evidence of tampering.
U.S. Pat. No. 3,209,569 teaches a tamper indicating casing for locks. A second lock housing is provided which completely encloses the lock structure so that access to the lock can only be obtained by breaking a seal. Observers of the lock and housing will then know that access has been obtained to the lock operating structure. It is an expensive apparatus and requires that the fracturable part be replaced every time that the lock is opened.
U.S. Pat. No. 4,074,547 shows an automobile ignition switch containing frangible elements to frustrate forced disassembly of the lock by a thief such as by forcing a screw into the keyway and then pulling the cylinder out with the screw and a pry bar or impact hammer. In this prior art, the frangible elements cause the cylinder to separate and the part that remains in the vehicle is further detented into locked position and has no holes into which a screw or other tool could be forced. This teaching does not permit the access that is necessary into a personal computer that has been sent to a repair depot without a key.
U.S. Pat. No. 4,413,493 is directed to a frangible lock actuator for an automotive vehicle trunk which like 4,047,547 permits the cylinder to be forcibly extracted. This teaching leaves only a virtually ungraspable nub held into the lock by two spring washers in the nearly inaccessible cylinder opening. The nub shaft prevents use of a screw driver to then open the car trunk.
U.S. Pat. No. 4,426,858 is also similar to the above references in that access to the locked area is frustrated by a flexible link between the lock cylinder and the latch. When the cylinder is extracted or pushed in, the flexible link frustrates latch rotation and further, the latch actuator becomes disassembled preventing actuation of the latch.
U.S. Pat. No. 4,685,317 teaches a frangible plug which is inserted into a key hole to prevent insertion of a key. The plug has teeth and a frangible score line which allows an authorized person possessing a special tool to destroy and remove the plug as shown in FIG. 5 of this patent. A plug will not solve in the instant need for a lock that operates normally but can be forced by a repair person yet leaves clear evidence when forced by someone tampering with the computer.
U.S. Pat. No. 4,890,006 is a computer switch lock having a reset as well as off and on positions. It has no frangible elements and does not allow entry by a repair person without a key.